1. Field of the Invention
The present invention relates generally to a handoff method in a mobile communication system, and in particular, to a method for reducing a handoff time.
2. Description of the Related Art
FIG. 1 illustrates a network configuration of a conventional mobile communication system. This network configuration is commonly applied to IS (Interim Standard)-95A/B, GSM (Global System for Mobile Communications), IS-2000, WCDMA (Wideband Code Division Multiple Access), and UMTS (Universal Mobile Telecommunication Service) systems, although the names of the components may be different in each system.
Referring to FIG. 1, a mobile terminal (MT) 101 represents a mobile communication terminal. The mobile terminal 101 may serve as either a voice-based legacy terminal or an IP terminal supporting IP (Internet Protocol). A base transceiver station (BTS) 102 manages radio resources, and actually exchanges data packets and control information with the mobile terminal 101 located in its service area (or cell) over a radio link. A base station controller (BSC) 103, which controls BTSs 102-A and 102-B, supports a signaling protocol for call setup and release. A GW/MSC (Gateway/Mobile Switching Center) 105, which connects its own network to a mobile communication network, the Internet, a PSTN (Public Switched Telephone Network) and a PSDN (Public Switched Data Network), supports protocol translation between different networks. As a logical name, the gateway GW can be called a PDSN (Packet Data Service Node), an AGW (Access Gateway) or an MGW (Media Gateway).
In the mobile communication network configuration, a link between the BSC/BTS 103/102 and the GW/MSC 105 can be formed with either a circuit network constructed using a leased line such as E1/T1 like the existing mobile communication network, or an IP packet network constructed using an IP router. In the former case, the BSC 103 is connected to the BTSs 102-A and 102-B with E1/T1, and IP is used as an upper transmission layer. In the latter case, the BSC 103 and the BTSs 102-A and 102-B are separately connected to an IP network through, for example, a router, instead of being directly connected.
FIG. 2 illustrates a detailed structure of the BSC 103 illustrated in FIG. 1. Referring to FIG. 1, a BSC main controller 213 manages resources of the BSC 103 and partial resources of the BTSs 102-A and 102-B, and controls the overall operation of the BSC 103. A first interface 223 interfaces a signal between the GW 105 and the BSC 103. An intra-BSC switch 233 is a router for managing routing and switching functions within the BSC 103. A second interface 243 interfaces a signal between the BSC 103 and the BTS 102-A and 102-B. Here, the first interface 223 and the second interface 243 each use NIC (Network Interface Card) or LIC (Line Interface Card) for connection with the GW 105 and the BTSs 102-A and 102-B, respectively. An SDU/RLP (Service Data Unit/Radio Link Protocol) processor 253 exchanges traffic with the mobile terminal 101. Here, an SDU is a given data unit distinguished by the service type, and an RLP is a protocol for radio transmission of data. A data packet transmitted by the RLP is called an “RLP packet.”
FIG. 3 illustrates a detailed structure of the BTSs 102-A and 102-B shown in FIG. 1. Both 102-A and 102-B have the same structure. Therefore, only a description of BTS 102-A will be given herein below.
Referring to FIG. 3, a BTS main controller 312 manages wire/wireless resources of the BTS 102-A, and controls the overall operation of the BTS 102-A. A first interface 322 interfaces a signal between the BSC 103 and the BTS 102-A. An intra-BTS switch 332 is a router for managing routing and switching functions within the BTS 102-A. An RF (Radio Frequency) scheduler 342 is a processor for scheduling packet transmission in order to efficiently use radio resources in the BTS 102-A. The RF scheduler 342 can be realized with either a separate board or a part of channel cards 352-1 to 352-N. The channel cards 352-1 to 352-N, together with the SDU/RLP processor 253 in the BSC 103, code and spread data transmitted to the mobile MT 101, or despread and decode a signal received from the MT 101. An RF device 362 up-converts baseband signals from the channel cards 352-1 to 352-N and provides its output to the MT 101, or down-converts an RF signal received from the MT 101 and provides its output to a corresponding channel card.
Such a mobile communication system divides its entire service area into a plurality of cells, and services the individual cells using a plurality of BTSs. In an actual radio environment, the cells partially overlap with one another, and in the overlapping region, a mobile terminal may receive signals from two or more BTSs. In such a cellular mobile communication system, in order to maintain a call of a mobile terminal moving between cells, a handoff procedure for exchanging call control signals between BTSs is required. The handoff is performed by a BSC that controls the BTSs. A conventional handoff procedure performed in the mobile communication system will be described herein below.
FIG. 4 illustrates a conventional handoff procedure according to the prior art. In FIG. 4, a handoff occurs when an MT in communication with a serving BTS#1 moves into a cell of another BTS#2. In this case, BTS#1 is called a source BTS, and BTS#2 is called a target BTS. In step 401, the MT is located in a service area, or a cell, of the source BTS, and a BSC controls only a communication path between the source BTS and the MT. The BSC transmits data traffic for the MT only to the source BTS. In step 403, RLP packets from the BSC are transmitted to the MT through the source BTS. Sequence numbers of the RLP packets are shown in brackets.
As the MT moves and enters a service area, or a cell, of the target BTS, the MT may receive a signal from the target BTS. Then, in step 405, the target BTS is registered as a candidate member of an active set for the MT. Even in this process, RLP packets from the BSC are transmitted to the MT through the source BTS, in step 407.
When the MT gets closer to the target BTS, a signal received from the target BTS becomes higher than a predetermined handoff threshold. Then, in step 409, the MT is handed off from the source BTS to the target BTS under the control of the BSC. As a result, in step 411, RLP packets from the BSC are transmitted to the MT through the target BTS.
In step 413, as the MT moves away from the source BTS, the BSC drops the source BTS from a handoff candidate set for the MT. In step 415, the BSC transmits RLP packets only to the target BTS. Then, in step 417, the MT receives RLP packets transmitted from the BSC through the target BTS.
FIG. 5 illustrates a detailed description of that procedure performed in step 409 of FIG. 4. In step 409, an MT switches a serving BTS from a source BTS to a target BTS. The BSC may detect that the MT switches a serving BTS from a source BTS to a target BTS (i.e., a handoff occurs), either through the BTSs or by itself. It is assumed in FIG. 5 that the BSC detects the occurrence of a handoff by itself. In step 501, the BSC transmits to the source BTS a sequence retrieve message VSHO(Virtual Soft Hand-Off)_Sequence_Retrieve MSG for retrieving a sequence number (i.e., the last sequence number) of an RLP packet that was last transmitted to the MT. In response to the message, the source BTS transmits to the BSC a sequence notification message VSHO_Sequence_Notification MSG for notifying the sequence number of the RLP packet that was last transmitted to the UE in step 503.
Referring to FIG. 4, when the target BTS (BTS#2) was registered as a candidate BTS, the source BTS (BTS#1) has previously received RLP packets with sequence numbers of 7, 8, and 9 from the BSC, and transmitted only the RLP packet with the sequence number of 7 to the MT. At the request of the BSC, the source BTS (BTS#1) notifies the BSC that the sequence number of the RLP packet that was last transmitted to the MT is 7. After a handoff is performed, the BSC transmits RLP packets with sequence numbers of 8 or larger, to the MT through the target BTS (BTS#2).
If the BSC is notified through the BTSs that the MT has been handed off, the source BTS may transmit the message VSHO_Sequence_Notification MSG for notifying the sequence number of the last transmitted RLP packet to the BSC.
A format of the sequence retrieve message VSHO_Sequence_Retrieve MSG transmitted from the BSC to the source BTS is illustrated in FIG. 6. As illustrated, the sequence retrieve message includes an MSG_TYPE field indicating the type of message, a CODE field including a control code, a LENGTH field indicating a length of the message, and a USER/FLOW-ID field for identifying a user or data flow.
In addition, a format of the sequence notification message VSHO_Sequence_Notification MSG transmitted from the source BTS to the BSC as a response message is illustrated in FIG. 7. As illustrated, the sequence notification message includes an MSG_TYPE field indicating the type of message, a CODE field including a control code, a LENGTH field indicating a length of the message, a USER/FLOW-ID field for identifying a user or data flow, and a LAST-XMITED-RLP-SEQUENCE field indicating a sequence number of an RLP packet that was last transmitted to the MT.
FIG. 8 is a flowchart illustrating a conventional handoff procedure performed by the BSC according to the prior art. Referring to FIG. 8, the BSC waits for a handoff in step 801, and determines whether a handoff occurs in step 803. If a handoff does not occur, the BSC returns to step 801 and waits for occurrence of a handoff. Otherwise, if a handoff occurs, the BSC transmits a sequence retrieve message VSHO_Sequence_Retrieve MSG for retrieving a sequence number (i.e., the last sequence number) of an RLP packet that was last transmitted to the MT, to the source BTS in step 805. Thereafter, the BSC waits for a response to the transmitted message in step 807, and determines in step 809 whether a response message is received from the source BTS. Upon receiving a response message VSHO_Sequence_Notification MSG, the BSC forwards RLP packets with the last sequence number and its succeeding sequence numbers, to the target BTS in step 811. The target BTS then forwards the next packets provided by the BSC to the MT.
FIG. 9 is a flowchart illustrating a conventional handoff procedure performed by the source BTS according to the prior art. Referring to FIG. 9, the source BTS waits for a sequence retrieve message VSHO_Sequence_Retrieve MSG for retrieving a sequence number (i.e., the last sequence number) of the last transmitted RLP packet, from the BSC in step 901, and determines whether the sequence retrieve message is received in step 903. If the sequence retrieve message is received, the source BTS retrieves the last sequence number from a predetermined memory area, in step 905. Thereafter, in step 907, the source BTS transmits a sequence notification message VSHO_Sequence_Notification MSG with the retrieved last sequence number to the BSC.
The steps 901 and 903 of FIG. 9 are performed when the BSC detects a handoff of the MT by itself. As another example, the source BTS may personally notify the BSC of the last sequence number. In this case, the steps 901 and 903 are unnecessary. That is, if the source BTS receives a handoff request from the BSC or detects by itself that the MT enters a handoff region, then the source BTS notifies the BSC of a sequence number of the RLP that was last transmitted form the source BTS to the MT.
As described above, in the prior art, a handoff delay is generated by performing an handoff by MT, detecting an occurrence of the handoff by a BSC, notifying the BSC of a sequence number of the last RLP packet transmitted from a source BTS to the MT, transmitting a new RLP packet to a target BTS by the BSC based on the notified sequence number, and resuming a call with the MT by the target BTS. As a result, an RLP exchange between the MT and the BSC is suspended during the process stated above.
In the future mobile communication system, an MT will rapidly select a BTS having a good radio environment. In this case, the handoff delay may disturb a normal call. For example, while a BSC performs the handoff procedure as an MT moves from a BTS_A to a BTS_B, the MT may move again from the BTS_B to another BTS_C. In this case, continuous call congestion may occur, causing call interruption between the MT and the BSC. In particular, because it is very important for the future mobile communication system to secure QoS (Quality of Service), there is a demand for a new technique for reducing a handoff time and increasing QoS during a handoff.